Trends in performance requirements call for increased data rates and use of wideband RF signals in many electronic systems, including electronic warfare (EW) applications. Moreover, electromagnetic immunity (EMI) requirements also point towards the deployment of fiber optic links on defense and other sensitive aerial platforms. These platforms—which include tactical platforms and surveillance aircraft—typically require at least 10 GHz of receiver coverage for radar warning receivers, self protection jammers, and electronic attack receivers. The 10 GHz of instantaneous bandwidth (IBW) supported by the photonic ADCs disclosed herein should enable a single processor to capture and digitize signals up to 10 GHz without downconverters that add circuitry/filter requirements for the system. Aside from providing substantial performance enhancements in IBW for these platforms, the disclosed invention also supports antenna remoting functions.
On many airborne platforms hosting Radar Warning Receivers (RWRs) and RF counter measure (RFCM) subsystems, the antennas of the aircraft need to be positioned so that they can locate threat position and optimize signal reception. For example, on some military aircraft, the defensive antenna suites are located either in the tail boom or fuselage side fairings. These antennas then deliver signals to a receiver located in the main wheel center-bay. Presently, each antenna requires more than 10 ft of coaxial cables to connect it to a repeater, followed by several waveguide sections to complete the RF-path to the receiver. Fiber remoting will offer low loss, light weight, and EMI resistant connectivity from antenna to ADCs and digital receivers that may not be co-located with the antenna frontend. The efficiency and broadband advantages of fiber optics will also reduce the gain requirements for the antennas.
FIG. 1 depicts a diagram of a prior art Analog to Digital Converter (ADC) with a resolution of 7 Effective Number Of Bits (ENOB) for a 10 GHz analog input signal. In the photonic ADC shown in FIG. 1, two physically distinct dispersion compensating fibers (DCFs, denoted D2+, D231 ) are used to stretch the complementary RF-modulation (MZM+, MZM−) available from the Electro-Optic Modulator (EOM)'s outputs. This was evidently adopted to facilitate separation of MZM+ and MZM− signals for post-processing. As shown in FIG. 1, the two DCFs (for stretching the RF-modulation) were followed by two sets of EDFAs, WDM-demultiplexers and then four photodiodes that track the wavelengths λ1 and λ2 for MZM+ and MZM−, respectively, in this two channel Time-Stretch (TS) photonic ADC system.
The present invention should overcome several disadvantages associated with the prior art shown in FIG. 1. First, in FIG. 1, the two dispersion compensating fibers (DCFs) (with α˜0.4 dB/km), each being 180 km long the Mach Zehnder (MZ) EOM, introduce significant insertion loss that needs to be overcome via use of large gains in the Erbium Doped Fiber Amplifiers (EDFAs) following them. In the preferred embodiment of the disclosed invention, a Chirped Fiber Grating (CFG) that has intrinsic insertion losses of 1.5-2 dB while providing a dispersion D2 of ˜2000 psec/nm is used instead. Second, because two physically distinct dispersion elements D2+, D2− and EDFAs are used for the complementary signals MZM+ and MZM−, two sets of post-processing algorithms are needed to correct for higher order distortions (“time warp”—see Gupta and B. Jalali, “Time-Warp correction and calibration in photonic time-stretch analog-to-digital convert”, Optics Letters, Vol. 33, No. 22, 2008), mismatches in third order dispersion coefficients (or ones of even higher order) between D2 and D1, or small optical nonlinearities (due to high peak powers) in the fibers. In the preferred embodiment of the disclosed invention, only a single dispersion element is used immediately downstream of the MZ EOM.
Finally, the Time-Stretch (TS) photonic ADC system of FIG. 1, uses single element photodiodes for the photodetection of I1 (corresponding to MZM+) and I2 (corresponding to MZM−), so that the linear phase modulation φ(t) of the EOM can be recovered. In the process, the noise floors of the photodetector (PD) outputs suffer a significant degradation due to the presence of signal-spontaneous emission beat-noise that originates from the EDFAs. For EDFAs with a noise figure of NFOA and a gain of GOA , the above optical amplifier noise (NoiseOA), measured over a signal bandwidth Δf, is given by:NoiseOA=(NFOAGOAA)(2eIdRL)Δf   Eqn. 1
where A is the insertion loss of the passive optical elements (WDM3 and WDM4 in FIG. 1) between the EDFAs and the single element PDs, Id is their DC photocurrents, and RL is their load resistances. Because the prior art subsystem does not offer any common mode rejection of the relatively intensity noise derived from the laser source or the EDFAs (NoiseOA), the SNR of the photodetector output is degraded. Therefore, the achievable ADC resolution, measured in terms of ENOB, is reduced.
Other TS photonic systems are known in the prior art. See, for example, G. Sefler, J. Conway and G. Valley, “Wide Bandwidth, High Resolution Time-Stretch ADC Scalable to Continuous-Time Operation”, Conference on Lasers and Electro-Optics (CLEO) 2009, Baltimore and R. Walden, “Analog-to-Digital Conversion in Early Twenty-First Century”, Wiley Encyclopedia of Computer Science and Engineering, John Wiley & Sons, 2008.